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1. Mid-IR spectroscopy of Er ³⁺ doped low-phonon CsCdCl ₃ and CsPbCl ₃ crystals

   https://doi.org/10.1364/JOSAB.470152  

   Brown, Ei Ei; Fleischman, Zackery D.; McKay, Jason; Hommerich, Uwe; Kabir, Al Amin; Riggins, Jazmine; Trivedi, Sudhir; Dubinskii, Mark (January 2023, Journal of the Optical Society of America B)

   The mid-IR spectroscopic properties of E r 3 + doped low-phonon C s C d C l 3 and C s P b C l 3 crystals grown by the Bridgman technique have been investigated. Using optical excitations at ∼ 800 n m and ∼ 660 n m , both crystals exhibited IR emissions at ∼ 1.55 , ∼ 2.75 , ∼ 3.5 , and ∼ 4.5 µ m at room temperature. The mid-IR emission at 4.5 µm, originating from the 4 I 9 / 2 → 4 I 11 / 2 transition, showed a long emission lifetime of ∼ 11.6 m s for E r 3 + doped C s C d C l 3 , whereas E r 3 + doped C s P b C l 3 exhibited a shorter lifetime of ∼ 1.8 m s . The measured emission lifetimes of the 4 I 9 / 2 state were nearly independent of the temperature, indicating a negligibly small nonradiative decay rate through multiphonon relaxation, as predicted by the energy-gap law for low-maximum-phonon energy hosts. The room temperature stimulated emission cross sections for the 4 I 9 / 2 → 4 I 11 / 2 transition in E r 3 + doped C s C d C l 3 and C s P b C l 3 were determined to be ∼ 0.14 × 10 − 20 c m 2 and ∼ 0.41 × 10 − 20 c m 2 , respectively. The results of Judd–Ofelt analysis are presented and discussed. 

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2. Evaluating mid-IR laser potential of Er ³⁺ vs. Dy ³⁺ in low-maximum-phonon-energy hosts for 4.1-4.8 µm applications

   https://doi.org/10.1364/OME.572481  

   Brown, Ei Ei; Fleischman, Zackery D; Merkle, Larry; Hommerich, Uwe; McKay, Jason; Palosz, Witold; Trivedi, Sudhir; Dubinskii, Mark (January 2025, Optical Materials Express)

   Andrea, A (Ed.)

   A comparative spectroscopic investigation was conducted to evaluate the potential of Dy3+ and Er3+ doped Ga2Ge5S13 chalcogenide glass and CsCdCl3 crystal for suitability as gain media for mid-infrared lasers operating within the 4.1-4.8 um spectral range. Both rare-earth ions exhibited broad mid-IR emissions at room temperature centered at ∼4.4-4.5 µm, attributed to the Dy3+: 6H11/2→H13/2 and Er3+:4I9/2→4I11/2 transitions. The fluorescence lifetimes of these mid-IR transitions varied significantly between these two host materials. Dy3+and Er3+ doped CsCdCl3 crystals exhibited long lifetimes of ∼9-12 ms, while the same ions in Ga2Ge5S13 glass yielded lifetimes in the range of ∼1 ms. Temperature-dependent lifetime measurements further confirmed the role of multiphonon relaxation in Dy3+: Ga2Ge5S13, although fitting with the temperature dependence of the multiphonon energy-gap law indicated the presence of additional nonradiative decay channels, likely involving impurity interactions. In contrast, Er3+ emissions exhibited minimal temperature dependence across both hosts, with measured lifetimes closely matching calculated radiative values. Judd-Ofelt analyses revealed radiative properties and transition probabilities comparable to other low-maximum-phonon-energy rare-earth-doped hosts, with the studied Dy3+ doped host materials standing out by offering sigma-tau products several times higher than those of Er3+in the same hosts. 

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3. Spectroscopic Evaluation of Er: Ga2Ge5S13 Glass for Mid-IR Laser Applications

   E. Brown; Z. D. Fleischman; J. McKay; U. Hommerich; W. Palosz; S. Trivedi; and M. Dubinskii (July 2022, Optical materials express)

   We present mid-IR spectroscopic characterization of the low-phonon chalcogenide glass, Ga2Ge5S13 (GGS) doped with Er3+ ions. Under the excitation at ∼800 nm, Er3+:GGS exhibited broad mid-IR emission bands centered at ∼2.7, ∼3.5, and ∼4.5 µm at room temperature. The emission lifetime of the 4I9/2 level of Er3+ ions in GGS glass was found to be millisecond-long at room temperature. The measured fluorescence lifetimes were nearly independent of temperature, indicating negligibly small nonradiative decay rate for the 4I9/2 state, as can be expected for a low-maximum-phonon energy host. The transition line-strengths, radiative lifetimes, fluorescence branching ratios were calculated by using the Judd-Ofelt method. The peak stimulated emission cross-section of the 4I9/2 → 4I11/2 transition of Er3+ ion was determined to be ∼0.10×10−20 cm2 at room temperature. 

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4. Comparative study of rare-earth doped low-phonon fluoride and chloride crystals for mid-IR laser potential

   E. Brown, Z. D. (April 2021, Proceedings SPIE 11723, Laser Technology for Defense and Security XVII)

   A comparative study was performed on the mid-infrared emission properties of trivalent erbium (Er3+) and holmium (Ho3+) doped fluorides (BaF2, NaYF4) and ternary chloride-based crystals (CsCdCl3, KPb2Cl5,). All crystals were grown by vertical Bridgman technique. Following optical excitation at 800 nm, all Er3+ doped fluorides and chlorides exhibited mid-infrared emissions at ~4500 nm at room temperature. The mid-infrared emission at 4500 nm, originating from the 4I9/2 -> 4I11/2 transition, showed long emission lifetime values of ~11.6 ms and ~3.2 ms for Er3+ doped CsCdCl3 and KPb2Cl5 crystals, respectively. In comparison, Er3+ doped and BaF2 and NaYF4 demonstrated rather short lifetimes in the microsecond range of ~47 us and ~205 us, respectively. Temperature dependent decay time measurements were performed for the 4I9/2 excited state for Er3+ doped BaF2, NaYF4, and CsCdCl3 crystals. We noticed that the emission lifetimes of Er3+:CsCdCl3 were nearly independent of the temperature, whereas significant emission quenching of 4I9/2 level was observed for both Er3+ doped fluoride crystals. The temperature dependence of the multiphonon relaxation rate for 4.5 um mid-IR emissions was determined for the studied Er3+ doped fluorides using the well-known energy-gap law. Using ~890 nm excitation, all studied Ho3+ doped fluorides and chlorides exhibited mid-infrared emissions at ~3900 nm originating from the 5I5 -> 5I6 transition. The longest emission lifetime of the 5I5 level was determined to be ~14.55 ms from the Ho3+:CsCdCl3 crystal. The room temperature stimulated emission cross-sections for the Er3+ 4I9/2 -> 4I11/2 and Ho3+ 5I5 -> 5I6 transitions were determined using the Füchtbauer-Landenburg equation. Among the studied crystals, Er3+ doped chlorides are more than two orders of magnitude better in terms of emission lifetimes and sigma-tau product than the fluoride crystals. 

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5. Materials development and mid-infrared emission properties of Dy-doped TlPb2Br5 and CsPbCl3

   U. Hommerich, D. Hart (January 2020, Proc. SPIE 11276, Optical Components and Materials XVII)

   We report on the materials development and infrared (IR) optical spectroscopy of Dy3+-doped into the ternary lead halides compounds TlPb2Br5 and CsPbCl3 for photonic applications. The investigated Dy-doped ternary lead halides were synthesized from purified starting materials followed by melt-growth using vertical Bridgman technique. Under optical pumping at 1.35 m (6H15/26H9/2+6F11/2), broad mid-IR emissions in the ~2.9 µm (6H13/26H15/2), ~4.3 µm (6H11/26H13/2), and ~5.4 µm (6H9/2 +6F11/2 6H15/2) regions were observed at room-temperature. Compared to Dy: TlPb2Br5, the mid-IR emission from Dy: CsPbCl3 was significantly weaker suggesting the existence of non-radiative losses. Temperature dependent lifetime studies and a Judd-Ofelt analysis were performed for Dy: TlPb2Br5. 

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